I'm want to define = operator in a class to change values of an array inside that class.
Is there anyway to return array's index by reference in mql5? If not, is there any way to define = operator to change values of that array inside the class?
Since MTx doesn't have pointers (only handles to classes,) it can't be done directly with POD.
If you have operator returning the internal array element, then no.
if you have operator return a helper class (containing pointer to the container class and the index) then the helper class can convert the assignment (*helper=value) to container.put(index,value). But then you can't do Print(container[x]) but instead Print(container.get(x))
IMO, it's not worth it. Just define container.put(index,value) and container.get(index) and be done.